1. Field of the Invention
This present invention relates generally to electrical circuitry for gas plasma systems for the surface modification of a workpiece and more particularly to circuitry for multiple electrode gas plasma reactors.
2. Description of Prior Art
Gas plasma modification of surfaces has been found to be quite useful for a multitude of applications crossing many fields. These surface modifications include, but are not limited to, etching (metals, dielectrics, polymers, etc.), deposition (metals, dielectrics, polymers, etc.), etchback and desmear of printed circuit boards, and chemical surface treatment (including cleaning). A common problem which is encountered in plasma processing is the difficulty of obtaining uniform process results across the reactor. This problem is particularly evident in reactors which have multiple process areas utilizing a plurality of electrodes as was demonstrated by James W. Wilson, Plasma Etching of Organic Materials in Large Multicell Reactors, Electrochemical Society Extended Abstracts, 84-2, Abstract No. 369, Page 521, 1984 in a reactor used to desmear and etchback printed circuit boards.
One cause of this phenomena is the difference in impedance in each of the electrical feeds to the driven electrodes brought about by having electrode feeds which are not identical or symmetrical to each other. This results in a non-symmetrical distribution of power among the electrodes which manifests itself in a non-uniform plasma and thus non-uniform processing.
U.S. Pat. No. 4,381,965 illustrates a plasma etching reactor which has multiple pairs of electrodes which do not have symmetrical feeds. Each driven electrode receives the RF excitation by way of separate variable capacitances each in series with the input to the electrode plates. Uniform etching performance in each plasma region is achieved empirically by tuning each of the latter capacitances, determining the degree of etching occurring in each plasma region, re-adjusting the capacitances, observing the changes and continuing the testing and re-adjusting procedures until the etching effects are balanced. This process is lengthy and can be quite arduous. In addition, these settings are only true for a very narrow range of process parameters (power, pressure, gas utilized, load, etc.) and must be repeated when the parameters are changed significantly.
Another cause of non-uniform plasmas is the change in the electrical characteristics of the plasma due to loading effects, i.e., placement of multiple parts within a reactor. In U.S. Pat. No. 4,282,077, which describes a plasma reactor (a multiple electrode system) used for desmear and etch-back of printed circuit boards, this problem was overcome by utilizing variable inductances in series with each of the electrodes. Again uniformity is only obtained emperically using a lengthy and arduous procedure of tuning individual inductors that is similar to the one mentioned above in U.S. Pat. No. 4,381,965. In addition, the variable inductors are positioned within the reactor and thus exposed to the plasma conditions where they are subject to the atmospheric permeability variation as the plasma's electrical characteristics are changed. This feature makes a wide pressure use at a single setting difficult if not impossible to achieve. The gas chemistry and the process conditions which affect the electrical characteristics of the plasma also contribute to this variability.
Another example of a multiple electrode system is demonstrated by U.S. Pat. No. 4,474,659 where the workpieces themselves are acting as electrodes and multiple generators are used to obtain good uniformity. In this case, however, the goal is not to achieve electrical equality among the electrodes, but to vary the RF excitation power to each individual electrode to compensate for differences in gas composition and flow between various electrodes. In this patent is also disclosed a planar type electrode arrangement which is composed of multiple areas driven by different generators or by a single generator with attenuation of the individual feeds to the different areas of the electrode.